It is known to discharge a bead of hot melt adhesive from a nozzle in a spiral pattern so that, for example, the bead is deposited in a series of overlapping loops. Such nozzles typically incorporate a plurality of air discharge passages surrounding an adhesive discharge passage. The air discharge passages direct so-called pattern air toward the discharged adhesive to cause it to take on a specific configuration or pattern on a substrate. When there is relative perpendicular movement between the adhesive bead and an underlying substrate, for example, a pattern of overlapping adhesive loops may be deposited on the substrate. Various apparatus and methods exist for applying liquids such as hot melt adhesives in overlapping, generally circular swirl patterns or other patterns using pressurized streams of air.
In prior dispensing apparatus, the pattern air was typically channeled from an air source through air discharge passages surrounding the adhesive discharge outlet. The air discharge passages have been disposed adjacent to the adhesive discharge passage and in direct thermal communication therewith. As such, the temperature of the structure forming the air discharge passages has been substantially equal to that of the adhesive discharge passage and the adhesive. Unfortunately, if pattern air at relatively cool temperatures, such as ambient temperature, is circulated through the air discharge passages or other air passageways in the apparatus, the adhesive discharge outlet and other adhesive passageways can be cooled to unsatisfactory levels. Specifically, relatively cool pattern air can carry significant amounts of heat away from the nozzle assembly through heat transfer as it moves through the air discharge passages or other air passageways in thermal communication with the hot melt adhesive discharge orifices and/or other adhesive passageways. This cooling effect can cause the adhesive viscosity to increase and thus adversely affect the deposition of the adhesive onto a substrate in the desired pattern or patterns.
To overcome the cooling effect, pattern air has been heated before its introduction into the nozzle assembly. It was found that to effectively minimize the cooling effect, the pattern air must be heated at least 25.degree. F. to 50.degree. F. higher than the target adhesive temperature at the adhesive discharge outlet, which is typically about 300.degree. F. The heated pattern air effectively resolved the cooling effect created by the ambient temperature pattern air, but produced offsetting disadvantages. For example, heating the pattern air above the adhesive temperature increases the complexity of the adhesive dispensing apparatus and increases the cost and labor involved with set-up and operation of the apparatus.
Adhesive or liquid dispensing apparatus of this general type which does not require heated pattern air would have several advantages over the prior designs. For instance, the time required to set up the adhesive dispensing apparatus would be reduced as the time needed to properly adjust the temperature of the air would be eliminated. Additionally, the cost of operation would be reduced due to the elimination of external heaters for the pattern air. Another advantage of using ambient temperature pattern air is that the air may desirably cool the extruded adhesive bead just prior to its contact with the substrate. Because cooler adhesive would contact the substrate, substrate burn-through caused by hot adhesive may be prevented and a thinner substrate could be used, for example, resulting in reduced material cost.
For at least these reasons, it would be desirable to provide a hot melt adhesive or liquid dispenser capable of using ambient temperature pattern air or pattern air that at least does not have to be heated to a temperature approaching the hot melt adhesive or liquid temperature.